This invention relates generally to electrophoretically developing a latent electrostatic image contained on a photoconductive surface of an electrophotographic member with a liquid toner suspension, and more particularly concerns an improved electrophoretic toning system for an electrophotographic imaging apparatus.
Electrophotographic imaging apparatus normally includes a plurality of functional stations arranged in sequential proximity to an electrophotographic member, that is, to the photoconductive coating surface thereof. A surface charge potential is applied to the surface at a charging station.
The charged surface then is presented to an exposure or imaging station whereat light is projected thereto through a transparency forming a latent electrostatic charge image of the pattern carried by said transparency. The exposed surface then is brought to and past a toning or development station where toner suspension is applied to the electrostatic latent image carrying surface for rendering the same visible.
The image then is transferred to a transfer medium at the transfer station. After transfer, the photoconductive surface is cleaned of any residual toner and discharged to ground, said surface being returned to its initiate location for carrying out the same process but with a different transparency and, generally, a different toner.
The toner employed in the electrophoretic process can be viewed as a type of liquid ink comprising finely divided toner particles and a liquid dispersant. The conventional dispersant is an electrically insulating hydrocarbon, such as the isoparaffinic hydrocarbon liquid fraction sold in various grades by Exxon Company of Houston, Tex., under the trademark "ISOPAR". The toner particles dispersed in an insulating liquid may carry a positive or a negative polarity electrical charge. When the photoconductive surface is an n-type material such as cadmium sulfide, an electrical insulating liquid dispersant is employed wherein the toner particles carry a positive charge.
A description of one useful electrophotographic member which has high-speed and high resolution capability, is found in Kuehnle U.S. Pat. No. 4,025,339 of May 24, 1977. This member has a wholly inorganic, r.f. sputtered crystalline cadmium sulfide forming the photoconductive surface thereof.
Difficulties have been encountered in prior liquid toning systems in providing adequate density or darkness of the image color while maintaining the background of the image free of color. Image fogging is the condition referred to where, although the background of an image should be absolutely devoid of color some residual toner remains. Conventional electrostatic reproduction inherently has background fog because there is normally a surface noise field potential which attracts the toner particles. In order to obtain proper image density the toner must be uniformly deposited on the photoconductive surface of the electrophotographic member and the toner should be homogeneous. Additionally, the vapor point temperature of the toner should be such as to avoid producing excess evaporation.
The invention herein differs from prior liquid toning systems by providing means to enhance uniformity in the toner deposit while maintaining a homogeneous toner suspension. Accordingly, the toner suspension is agitated constantly and temperature rise of the toner is limited. It would be highly desirable to minimize fogging while providing a maximum density of the image to be toned. In the apparatus of the invention, of further advantage is the provision of toning means where precision machining is materially reduced, perhaps being required only for a planar development electrode.
The method and apparatus of the inventions is especially of value in imaging apparatus where multiple color toning is effected as for making color proofs. It thus is embodied in a module for single color apparatus and a plurality of toner modules for multiple color apparatus, each module having a different color associated therewith. Otherwise, the modules are identical and can be arranged side by side and adapted to be moved into functional position respectively on successive passes of an electrophotographic member relative thereto. The modules of an apparatus of this type can be removed individually for cleaning, servicing and replacement.